The present invention relates generally to processes for forming thin films and more specifically, relates to pulsed laser deposition methods using a desired target material for forming high quality thin films on a substrate.
Over the years, a number of different techniques have been developed and/or proposed for forming thin films on a substrate. As more and more applications for thin films are contemplated and new types of materials are discovered, there is a continuing need to produce high quality films. For example, the discovery of high Tc superconductor materials precipitated a great deal of research into the development of a practical method for making such materials. The primary practical application for such materials is the formation of thin films which may be used in certain instruments, such as SQUIDS and bolometers. Thin films formed of these materials may also be used for the general fabrication of superconductors in the form of thin films deposited onto wires or tapes. This is only one type of thin film application which requires a fabrication method which offers the precision and high quality necessary to produce such films.
Pulsed laser deposition (PLD) is a versatile deposition technique that has been in use for several years. The technique is based upon the vaporization of a small region of a target material by a high power laser. The technique has particular application in the deposition of oxide films, such as high temperature superconductors, ferrites, and ceramic films. The target material is then subsequently collected on a substrate in the form of a film deposited onto the substrate surface. Typically, the technique employs a series of very short (nanosecond duration) pulses, principally from a laser source which ablates the surface of the target material and then by using various means, the target material is deposited as a thin film on the substrate. The PLD method offers many advantages over other types of techniques for forming a thin film on a substrate. For example, the PLD method offers ease of deposition and the formation of crystalline films with good adhesion at low temperatures, even as low as room temperature. Another advantage of the PLD technique is the ability to reproduce the stoichiometry of the target in the film, including that of the multi-component targets. PLD is desirable for routine deposition at room or higher temperatures providing high quality crystalline thin films.
PLD is an excellent method for use in superconductor film growth processes and other coating processes for forming high quality thin films. PLD involves laser ablation and evaporation of a target material by a high power laser. The ablated material forms a plume comprising both undesirable large particles and desirable atoms and ions which all get deposited on a substrate. More specifically, the plume includes ions, electrons, atom clusters, and larger particulates of varying sizes. The plume extends from the target in a direction outward from the target. Often, the substrate is positioned so that it is directly in front of the target, at a distance of a few inches. Thus, the plume spreads onto the substrate to form the thin film. In this arrangement, the direct plume has a range of atom clusters and particulate sizes. The substrate may also be placed alongside the plume to collect a greater percentage of atomic species but at a lower deposition rate.
One of the disadvantages of using the PLD technique is that undesirable particulates form a part of the plume and are directed onto the substrate. These particulates generally constitute the large particles which are present in the plume and have sizes on the order of between about 0.1 to about 5.0 xcexcm in size. The inclusion of this size of particles in the thin film disadvantageously limits PLD commercialization. Most of the conventional PLD methods disadvantageously produce a particle density of about 400 particles per cm2.
The laser ablation of the target material also results in the creation of charged and neutral species having a varying degree of sizes. Only species of atomic dimensions of the target material are desired to be deposited on the substrate to form the thin film. If larger sized particulates form on the substrate, these particulates limit the uniformity of the deposited thin films and its applications. These particulates are formed as a result of a number of factors relative to the target. More specifically, the target may include a protruding surface; the target may have micro-cracks that are mechanically dislodged due to laser induced thermal or mechanical shock during the laser ablation process. In addition, larger particulates may be formed as a result of rapid expulsion of trapped gas bubbles beneath the target surface during laser irradiation and also the splashing of molten layers of the target material may result in the formation of larger particulates.
The presence of larger particulates in the thin film has serious ramifications in some specialized applications, such as tribological applications. In these type of applications, it is desirable to deposit coatings with very high hardness on precision bearings. These hard coatings can protect the steel surfaces of the bearings from wear and thereby improve the lifetime of the bearings. By increasing the lifetime of the bearings, the performance of a variety of moving mechanical assemblies can be improved. For example, machinery and pump performance can be improved due to this improved wear. PLD is an excellent technique for depositing such hard coatings; however, the incorporation of larger, hard particulate material in the coatings negatively impacts the performance of the bearings as these materials often have abrasive-like properties. The presence of abrasive-like particles in the coating can act to deteriorate and destroy the coating. This results in the production of more debris and also to a loss of coating adhesion.
Another limitation of the PLD method is that it is difficult to scale up the deposition process to accommodate larger size substrates having a surface area of about 10 cm2. To grow large area uniform films requires that the substrates be moved to accommodate uniform deposition over a larger area than the physical plume size.
Accordingly, the predominant problem with PLD methods is the creation and deposition of large particulates that impose a limitation on the potential scope of applications for the PLD method. These and other disadvantages or problems are solved or reduced using the apparatus and method of the present invention.
According to the present invention, a method for forming high quality thin films using a pulsed laser deposition (PLD) system is provided. In one exemplary embodiment, the system includes a PLD chamber wherein a laser beam ablates a target material creating an ionized plasma plume of ions and electrons which is diverted and deposited onto a substrate using a confinement magnet followed sequentially by a deflection magnet.
More specifically, the ablation of the target creates a plume of atomic species atoms, ions, electrons, atomic clusters, and particulates of varying sizes. The target is disposed in close proximity to the confinement magnet so that the plume is directed into the confinement magnet. The confinement magnet generates a magnetic field parallel to the plume ejection direction from the laser target. This magnetic field counters the tendency for the plume to naturally diverge and therefore acts to focus and concentrate the plume as it travels away from the target.
The concentrated plume is then introduced into the deflection magnet which includes magnetic coils and serves to apply a magnetic field to deflect the electrons and accompanying ions to the substrate. In one embodiment, the deflection magnet is a tubular magnetic member having an opening extending therethrough for receiving the plume. The deflection magnet generates an axial magnetic field which is parallel to the laser plume ejection direction (similar to the confinement magnet). The deflection magnet has a bend formed therein at an end proximate to the substrate for directing the plume onto the substrate which is disposed away from a longitudinal axis extending through the target and the confinement magnet. The magnetic field generated in the deflection magnet, including the bent portion thereof, constitutes a mechanism for filtering the neutral, uncharged matter (e.g., the atomic clusters and the particulates) of the plume from the charged matter (the atomic species atoms and the ions). The uncharged matter is not influenced by the magnetic field and thus travels in a relatively linear trajectory from the target through the confinement and deflection magnets. In this manner, only the charged matter is deflected onto the substrate to form the thin film and the undesirable atomic clusters and particulates are not deflected onto the substrate.
In another aspect of the present invention, deflector plates are disposed within the deflection magnet and an electric field is generated across the plates. A negatively charged plate acts to repel negative electrons away from the outer curved wall of the deflection magnet and toward a positively charged plate on an opposite surface of the deflection magnet. Because the positive ions are attracted to the negative electrons, the ions are thus assisted, especially in the bent portion, in following the electrons along the magnetic field direction of the deflection magnet toward the substrate.
Accordingly, the present invention, provides a simple, relatively inexpensive, yet effective PLD method of forming extremely clean films with reduced particulate densities and size. This method favors useful film properties, such as crystallinity and good adhesion, event at room temperature, because it relies upon using high energy ions for the deposition. The method therefore has tremendous potential for applications where the substrate is thermally sensitive. The present method may be applied to the production of a film from a great number of materials.
Other features and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.